B25S Should I Go For A Small Capacity Increase

[LightningAndStarfire]

Yes, should have written cylinder head. Sorry.

[LightningAndStarfire]

Please google "formula for compression ratio" .

Results seem relevant.

[Spaceman]

As I see it. The original capacity is 247cc and the compression ratio is 9.5:1. The combustion chamber volume = swept volume (247cc) / compression ratio = 26cc.
A 10% overbore takes the swept volume to 271.7cc , so the new compression ratio is 271.7 / 26 = 10.45:1.
I'm sure someone will correct me if I'm wrong but it can't be far out.

The above formula is not quite right as compression ratio = (combustion chamber vol + swept vol) / combustion chamber vol. This gives a combustion chamber vol of 29 cc.

The difficulty in predicting the CR of the enlarged bore is that the piston shape may have changed and therefore the combustion chamber volume may be different. In fact, it's possible that the new piston shape may have been designed to give the same CR - without specific design info on the new piston, it's difficult to know

[DAVE BRADY]

Hi,

I worked out that the 'extra clearance space' under any cylinder head would be less than 1cc so probably of little consequence.

Dave.

[Derek996]

As I see it. The original capacity is 247cc and the compression ratio is 9.5:1. The combustion chamber volume = swept volume (247cc) / compression ratio = 26cc.
A 10% overbore takes the swept volume to 271.7cc , so the new compression ratio is 271.7 / 26 = 10.45:1.
I'm sure someone will correct me if I'm wrong but it can't be far out.

The above formula is not quite right as compression ratio = (combustion chamber vol + swept vol) / combustion chamber vol. This gives a combustion chamber vol of 29 cc.

The difficulty in predicting the CR of the enlarged bore is that the piston shape may have changed and therefore the combustion chamber volume may be different. In fact, it's possible that the new piston shape may have been designed to give the same CR - without specific design info on the new piston, it's difficult to know

On reflection I see that you are right. I just did it of the top of my head without fully thinking it out. I also assumed no change to the piston crown which would make a compression change by itself.